1. Field of the Invention
The present invention relates to a technique to determine a reference point which is an origin point in a displacement detector such as an encoder which detects displacement of an optical scale.
2. Related Background Art
A rotary encoder for measuring a rotation angle or a rotation speed of a disk-shaped optical scale, which is an example of a displacement detector, is disclosed in U.S. Pat. Nos. 5,026,985, 5,059,791, 4,263,506 and 4,477,189.
Japanese Laid-Open Patent Application No. 63-81212 discloses a rotary encoder which uses a cylindrical optical scale having a slit-like grating formed on a side thereof to measure a rotation angle thereof at a relatively high resolution with a simple construction. By the rotary grating being cylindrical, no alignment of two gratings (a rotary grating and a fixed grating) which has been required in the prior art is necessary, and a cancellation effect for a detection error due to the eccentricity of a rotating shaft is attained. Thus, the high precision and the simplicity in mounting are attained. Such advantage is achieved by providing a focusing optical system in the scale (in a hollow area) and projecting, by the focusing optical system, an image of the grating in a first region on the side of the scale to the grating in a second region on the opposite side of the scale to the first region with respect to the rotating shaft of the scale.
On the other hand, an improved encoder which uses a similar slit grating or a cylindrical optical scale having a convexo-concave grating with a regular sloped surface such as a V-shaped groove formed on a transparent cylindrical member is disclosed in pending U.S. application Ser. No. 634,045 filed on Dec. 26, 1990. It utilizes a principle of Talbot effect of the grating instead of the focusing optical system. According to such encoder, in addition to the effect of the prior art described above, it can further improve the simplification of the overall configuration, the volume reduction and the inertia reduction.
When the above encoder is used in various systems, it is desirable to determine a reference position or an origin point of the rotation. By obtaining an origin point signal, an encoder capable of detecting an absolute position is provided. Configurations to obtain the origin point signal are shown in the above patents.